This invention relates to the purification of waste gases, and more particularly to a method and apparatus for removing sulfur oxides and nitrogen oxides from the gaseous combustion products of sulfur-containing fossil fuel such as high sulfur coal, and to obtain by-products of commercial value which would at least partially recoup the cost of the purification apparatus and operation thereof.
Recent emphasis on air pollution control has severely limited the use of sulfur-containing coal as a source of fuel in electric generating plants, steam plants and the like, despite the fact that huge deposits of sulfur-containing coal are readily available in this country and could be substituted for the more expensive and relatively scarce alternative forms of energy such as low-sulfur coal, natural gas and petroleum. If the current stringent environmental controls an emission of sulfur oxides could be met, it would thus be possible to ameliorate energy requirements for many years by conversion to sulfur-containing coal in many stationary electrical generating and heating and power plants now using other forms of fuel, thus releasing petroleum, natural gas and low-sulfur coal for other uses to which they are better suited.
Reduction of sulfur oxide levels in flue gases to less than 100 parts per million before discharge to ambient atmosphere would go far toward making high sulfur coals acceptable as an alternative fuel source.
The problem of acid rain in some areas has received considerable publicity, and the burning of high-sulfur coal has been alleged to be a cause. Here again reduction of sulfur oxides in flue gases would have a salutary effect if in fact sulfur (and/or nitrogen) oxides are responsible for this problem.
Recent approaches to the use of high-sulfur coal as a fuel have included mixing powdered limestone with powdered coal, the calcium in the limestone reacting with the sulfor oxides resulting from combustion to form calcium sulfate, or gypsum. Aside from the fact that grinding of the coal into powder represents a substantial (and otherwise useless) expenditure of energy, the calcium sulfate byproduct has little commercial value, and it is apparently contemplated that it be disposed of as landfill. This overlooks the possibility that the landfill could decompose chemically after a few years to release heavy concentrations of sulfur oxides or other noxious sulfur compounds.
A number of proposals have been made to convert sulfur oxides in flue gases to sulfuric acid, which is of course a byproduct of value. However, these proposals generally proceed along the conventional lines of providing scrubbers with towers, fixed or mobile beds, or plates, and requiring spray nozzles, jets, pumps and the like, with resultant high initial cost and maintenance problems.
U.S. Pat. No. 4,158,702 is representative of the provision of a plurality of spray zones in which gases are contacted by a scrubbing medium in the form of droplets or mist.
U.S. Pat. No. 3,795,732 discloses a method for converting sulfur dioxide in combustion gases to sulfur trioxide by means of a vanadium pentoxide catalyst, absorbing the sulfur trioxide with lithium sulfate, and regenerating the lithium sulfate. It is alleged that reduction of sulfur oxides in stack gases to less than 130 p.p.m. can be obtained.
Fixed bed catalytic converters for removal of sulfur oxides from flue gases are disclosed in U.S. Pat. Nos. 3,436,192 and 3,501,897.
Apparatus for removal of sulfur dioxide from flue gases is disclosed in U.S. Pat. No. 4,099,925, wherein a gas dispersing plate and weir are provided for contacting the gases with an absorption liquid, together with an absorption liquid reservoir and a liquid-dropping zone.
U.S. Pat. No. 4,189,309 discloses a method of desulfurizing flue gases by means of two recirculating water systems separated by a secondary cooling stage. A further stage involves desorption of sulfur dioxide under vacuum conditions.
U.S. Pat. No. 4,120,669 discloses a process for removing sulfur oxides from waste gases wherein sulfur dioxide is partially converted to sulfur trioxide in a converter containing vanadium pentoxide as a catalyst, followed by scrubbing with a spray of concentrated sulfuric acid to absorb the sulfur trioxide. The remaining waste gases, still containing substantial levels of sulfur dioxide, are then passed to an ozone reactor to convert most of the sulfur dioxide to sulfur trioxide, again followed by scrubbing with concentrated sulfuric acid. It is alleged that sulfur oxides can be reduced to a level of about 100 p.p.m. Ozone is generated in an amount stoichiometrically equivalent to the sulfur dioxide remaining after the catalytic conversion and first scrubbing step.
U.S.D.C. Publication PB-209191 relates to a modified chamber process for making sulfuric acid from flue gases. Problems with excessive reaction times are pointed out, together with an apparent need to burn additional sulfur-free coal (for energy or as a source for nitrogen oxides as a catalyst).
U.S. Pat. No. 3,581,467, issued to the inventor in the present application, discloses a method and apparatus for atomizing and elevating liquids and entraining gases therewith having utility, inter alia, in the fields of air cleaning, chemical processes wherein gases and liquids are mixed, and scrubbing of process gases with liquids. In this patent an elongated, substantially vertical cylindrical duct is provided having orifices at the top and bottom thereof, the bottom orifice having a non-horizontal configuration, a liquid reservoir beneath the duct maintained at a level such that a portion of the non-horizontal bottom orifice is partially submerged, and fan means positioned closely above the orifice in axial alignment therewith for generating an upwardly spiralling vortex of gases within the duct with the rotational velocity of the outer rotating confines of the vortex being between 2,000 and 12,000 feet per minute. The rotational velocity of the vortex is sufficient to cause the liquid within the confines of the duct to be atomized and elevated. At the same time gases are drawn into the duct through the non-horizontal bottom orifice and entrained and admixed with the atomized liquid.
The present invention utilizes a modification of the method and apparatus of the aforesaid patent 3,581,467, as will be described in more detail hereinafter.
As is well known, sulfuric acid has been produced for many years from sulfur dioxide both by the chamber process and by the contact process. The chamber process involves conversion of sulfur dioxide to sulfur trioxide with nitrogen dioxide (NO.sub.2) as a catalyst at a temperature of about 60.degree. C. maximum. The sulfur trioxide is absorbed in water (or dilute sulfuric acid) to form sulfuric acid, and the nitrogen dioxide which has been reduced to nitric oxide (NO) can be recovered and reused. The contact process involves conversion of sulfur dioxide to sulfur trioxide in the presence of a platinum or vanadium pentoxide catalyst at a minimum temperature of about 150.degree. C. The sulfur trioxide, which is in the form of a mist, is absorbed in concentrated sulfuric acid.
It is a feature of the present invention that modifications of either the chamber process or the contact process, or a combination of both, can be used to reduce to low levels sulfur dioxide and nitrogen oxides in large volumes of combustion gases. Even at a sulfur dioxide level of about 0.3% in the combustion gases the volume to be removed is very small in relation to the total volume of gases which must be handled. This presents problems in sizing the apparatus and in providing sufficient time for chemical reactions to occur resulting in removal of the sulfur dioxide.
It is an object of the invention to provide a method and apparatus which overcome the above problems. It is a further object to provide apparatus in the form of a module which can be coupled in series with as many like modules as may be needed to treat any required volume of combustion gases.
According to the invention there is provided a process for removing sulfur oxides from the gaseous combustion products of sulfur-containing fossil fuel, said combustion products containing sulfur dioxide, nitrogen oxides and water vapor, comprising the steps of passing said gaseous combustion products into a first enclosed chamber, introducing a supply of atomic oxygen and air into said chamber, subjecting said combustion products, atomic oxygen and air to a plurality of confined, upwardly spiralling vortical movements within said chamber in series, each upwardly spiralling vortex being generated in such manner as to produce a rotational velocity in the outer rotating confines thereof ranging between 2,000 and 12,000 feet (610 to 3,660 meters) per minute, maintaining a reservoir of sulfuric acid in said chamber, each said vortex being of sufficient magnitude to rupture the surface tension of said sulfuric acid at rest, thereby causing said vortex to atomize and elevate said sulfuric acid and to entrain and intermix said gaseous combustion products, atomic oxygen and air therewith, impinging the mixture of atomized sulfuric acid, gaseous combustion products, atomic oxygen and air in said vortex against rapidly rotating surfaces whereby to discharge said mixture outwardly and downwardly to fall by gravity into said reservoir, discharging the remaining gases from said first chamber into at least one additional chamber connected in series therewith, and repeating said steps of subjecting said mixture to said plurality of confined, upwardly spiralling vortical movements, and impinging said mixture against rapidly rotating surfaces, in said at least one additional chamber, the dwell time of said mixture in said chambers being sufficient to permit substantially complete conversion of sulfur dioxide in said gaseous combustion products to sulfur trioxide and nitrogen oxides to nitrogen dioxide, and to absorb said sulfur trioxide in said sulfuric acid, whereby to reduce the sulfur oxides content of said mixture to a level of about 100 ppm.
Apparatus in accordance with the invention includes at least one of a heat exchanger and electrostatic precipitator through which said gaseous combustion products at elevated temperature are initially passed, at least one reaction chamber in which sulfur dioxide is converted to sulfur trioxide in the presence of atomic oxygen and absorbed in sulfuric acid, and a discharge to ambient atmosphere, at least one module in each said reaction chamber, each said module comprising a confining housing, a substantially vertical duct having orifices at the top and bottom thereof and centrally located in said housing, the top orifice being horizontal, the bottom orifice having a non-horizontal configuration such that a portion of said orifice is partially submerged in a reservoir of sulfuric acid in said housing, a plurality of fan blades positioned closely above said top orifice substantially in axial alignment with said duct, at least a section of each of said fan blades functioning in the manner of backwardly inclined centrifugal blower blades when rotated rapidly, baffle means dividing said housing into inlet and outlet sections, respectively, and directing incoming gaseous combustion products toward said non-horizontal bottom orifice in said inlet section while preventing intermingling of said incoming gaseous combustion products with gaseous combustion products which have been raised in an upwardly spiralling vortex through said duct, impacted against said rapidly rotating fan blades and discharged outwardly and downwardly in said outlet section to drain into said reservoir, and an outlet from said module shielded by said baffle means from said incoming gaseous combustion products.